1. Field of the Invention
The present invention relates to an apparatus for curing an adhesive resin (binder) impregnated in a filter element during manufacture of an oil filter or an air filter for an automobile.
2. Description of the Related Art
In general, a disposable type filter element is produced by rolling a predetermined length of a strip of a filter material into a cylindrical form after the strip has been corrugated in a continuous manner and mounting fittings such as a protector inside. The filter material composed of wood pulp and polyester staple must be reinforced by inter-fiber bonding with a binder of thermosettable phenolic resin, and a strip of a filter material is impregnated with the above resin before delivery from a strip maker. At this stage, however, the resin between the fibers has not been cured but only a solvent for the resin has been removed. After the resin has been cured and hardened by heat-processing the shaped material, the filter element is reinforced to obtain a desired bending strength thereof.
In the conventional process, a hot air circulation oven, as shown in FIG. 6, is utilized for heat-processing the filter element to cure the resin impregnated therein and to impart the desired strength. This oven is provided with a fan 2 within a housing 3 of the oven, which fan is driven by a motor 1 to circulate hot air within the oven. A pair of heaters 4 are disposed across the passage way of the hot air, and a temperature in the oven is always maintained at a predetermined value by regulating the heaters 4 by a thermocontroller (not shown). An exhaust opening 6 and a suction opening 7 are formed by partitions 5a, 5b, and 5c. A conveyor 8 is disposed horizontally through the housing 3 of the oven, on which conveyor 8 a plurality of curing pots 9, each accommodating a filter element 12, are deposited. As shown in FIG. 7, the pot 9 is formed of an outer tube 10 and an inner tube 11 secured coaxially to each other, and the uncured filter element 12 is accommodated in a space formed between the outer and inner tubes 10 and 11, to maintain the profile thereof in a stationary position during the curing process.
In the above oven, since the hot air can not easily penetrate the body of the filter element 12, a longer processing period, for example, about 20 min. at an environmental temperature of 180.degree. C., is required for curing the resin impregnated in the filter element 12. This is especially true when the filter element 12 has a higher density, as shown in FIG. 4. Thus, for a continuous curing process, the oven must be large in size so that a large number of filter elements, e.g., a thousand or more, can be deposited on the conveyor in order to realize an economical processing speed. This, of course, requires an enormous installation cost and a vast floor space, which, in turn, results in a high manufacturing cost of the filter element.